Chapter18 broadcasting

Chapter18broadcasting

contents

• Introduction• broadcast address• unicast versus broadcast• dg_cli function using broadcasting• Race conditions

Introduction

• Multicasting support is optional in IPv4 but mandatory in IPv6

• broadcasting support is not provided in IPv6• broadcasting and multicasting require UDP, they

do not work with TCP• internet application using broadcasting

– ARP, BOOTP, NTP(network time protocol),routing daemon

Figure18.1

broadcast address

• Subnet-directed broadcast address:{netid, subnetid,-1}– this address all the interface on the specified subnet– most common today

• All-subnet-directed brocast address:{netid, -1, -1}– this address all subnet on the specified network

• network-directed broadcast address:{netid, -1}– this is used on network that does not use subnetting

• limited broadcast address:{-1, -1, -1}or 255.255.255.255– datagram destined to this address must never be forwarded by a ro

uter

Figure 18.2

unicast versus broadcast

unicast versus broadcast

dg_cli function using broadcasting

• Modify dg_cli function=> this allow to broadcast to the standard UDP daytime server, and printing all the replies.– Servaddr.sin_port = htons(13);

Figure 18.5#include "unp.h"static void recvfrom_alarm(int);void dg_cli(FILE *fp, int sockfd, const SA *pservaddr, socklen_t servlen){

int n;const int on = 1;char sendline[MAXLINE], recvline[MAXLINE + 1];socklen_t len;struct sockaddr *preply_addr;

preply_addr = Malloc(servlen);

Setsockopt(sockfd, SOL_SOCKET, SO_BROADCAST, &on, sizeof(on));

Signal(SIGALRM, recvfrom_alarm);

while (Fgets(sendline, MAXLINE, fp) != NULL) {

Sendto(sockfd, sendline, strlen(sendline), 0, pservaddr, servlen);

alarm(5);

for ( ; ; ) {len = servlen;n = recvfrom(sockfd, recvline, MAXLINE, 0, preply_addr, &len);if (n < 0) {

if (errno == EINTR)break; /* waited long enough for replies */

elseerr_sys("recvfrom error");

} else {recvline[n] = 0; /* null terminate */printf("from %s: %s", Sock_ntop_host(preply_addr, servlen), recvline);

}} } }

static void recvfrom_alarm(int signo){

return; /* just interrupt the recvfrom() */}

Figure 18.5

The subnet-directed broadcast address of 206.62.226.63>udpcli 206.62.226.63hifrom ……………from ……………from …………….from ……………..from …………….from …………….Hellofrom …………….from ……………from …………….from ……………..from …………….from …………….

Race conditions• When multiple processes are accessing data that is

shared among them but the correct outcome depends on the execution order of the processes

• race conditions of a different type often exist when dealing with signals. Problem: a signal can normally be delivered at anytime while a program is executing

• A Race Condition (See Figure 18.5)– alarm(1)– sleep(1)

#include "unp.h"static void recvfrom_alarm(int);void dg_cli(FILE *fp, int sockfd, const SA *pservaddr, socklen_t servlen){

int n;const int on = 1;char sendline[MAXLINE], recvline[MAXLINE + 1];sigset_t sigset_alrm;socklen_t len;struct sockaddr *preply_addr;



Sigemptyset(&sigset_alrm);Sigaddset(&sigset_alrm, SIGALRM);




Blocking and Unblocking the Signal

alarm(5);for ( ; ; ) {

len = servlen;Sigprocmask(SIG_UNBLOCK, &sigset_alrm, NULL);n = recvfrom(sockfd, recvline, MAXLINE, 0, preply_addr, &len);Sigprocmask(SIG_BLOCK, &sigset_alrm, NULL);if (n < 0) {

if (errno == EINTR)break; /* waited long enough for replies */

elseerr_sys("recvfrom error");

} else {recvline[n] = 0; /* null terminate */printf("from %s: %s", Sock_ntop_host(preply_addr, servlen), recvline);

}} } }



The unblocking of the signal, the call to recvfrom, and the blockingof the signal are all independent system calls.Assume recvfrom returns with the final datagram reply and the signal is delivered between the recvfrom and the blocking of the signal

=> the next call to recvfrom will blcok forever

static void recvfrom_alarm(int signl) { had_alarm = 1; return;}

Sigprocmask(SIG_UNBLOCK, &sigset_alrm, NULL);

if (had_alarm == 1)

break;

n = recvfrom(sockfd, recvline, MAXLINE, 0, preply_addr, &len);

Sigprocmask(SIG_BLOCK, &sigset_alrm, NULL);

#include "unp.h"static void recvfrom_alarm(int);void dg_cli(FILE *fp, int sockfd, const SA *pservaddr, socklen_t servlen){

int n;const int on = 1;char sendline[MAXLINE], recvline[MAXLINE + 1];fd_set rset;sigset_t sigset_alrm, sigset_empty;socklen_t len;struct sockaddr *preply_addr;

preply_addr = Malloc(servlen);Setsockopt(sockfd, SOL_SOCKET, SO_BROADCAST, &on, sizeof(on));FD_ZERO(&rset);Sigemptyset(&sigset_empty);Sigemptyset(&sigset_alrm);Sigaddset(&sigset_alrm, SIGALRM);


while (Fgets(sendline, MAXLINE, fp) != NULL) {Sendto(sockfd, sendline, strlen(sendline), 0, pservaddr, servlen);

Blocking and Unblocking the signal with pselect

Sigprocmask(SIG_BLOCK, &sigset_alrm, NULL);alarm(5);for ( ; ; ) {FD_SET(sockfd, &rset);n = pselect(sockfd+1, &rset, NULL, NULL, NULL, &sigset_empty);if (n < 0) {if (errno == EINTR)break;elseerr_sys("pselect error");} else if (n != 1)err_sys("pselect error: returned %d", n);

len = servlen;n = Recvfrom(sockfd, recvline, MAXLINE, 0, preply_addr, &len);recvline[n] = 0; /* null terminate */printf("from %s: %s",Sock_ntop_host(preply_addr, servlen), recvline);} } }



Figure 18.8#include "unp.h"

int pselect(int nfds, fd_set *rset, fd_set *wset, fd_set *xset,const struct timespec *ts, const sigset_t *sigmask)

{int n;struct timeval tv;sigset_t savemask;

if (ts != NULL) {tv.tv_sec = ts->tv_sec;tv.tv_usec = ts->tv_nsec / 1000; /* nanosec -> microsec */}

sigprocmask(SIG_SETMASK, sigmask, &savemask); /* caller's mask */n = select(nfds, rset, wset, xset, (ts == NULL) ? NULL : &tv);sigprocmask(SIG_SETMASK, &savemask, NULL); /* restore mask */

return(n);}

Using sigsetjump and siglongjump#include "unp.h"#include <setjmp.h>

static void recvfrom_alarm(int);static sigjmp_buf jmpbuf;

void dg_cli(FILE *fp, int sockfd, const SA *pservaddr, socklen_t servlen){

int n;const int on = 1;char sendline[MAXLINE], recvline[MAXLINE + 1];socklen_t len;struct sockaddr *preply_addr;






alarm(5);for ( ; ; ) {

if (sigsetjmp(jmpbuf, 1) != 0)break;

len = servlen;n = Recvfrom(sockfd, recvline, MAXLINE, 0, preply_addr, &len);recvline[n] = 0; /* null terminate */printf("from %s: %s",

Sock_ntop_host(preply_addr, servlen), recvline);}

}}

static voidrecvfrom_alarm(int signo){

siglongjmp(jmpbuf, 1);}

Figure 18.9

Using IPC from signal handler to function#include "unp.h"static void recvfrom_alarm(int);static int pipefd[2];

void dg_cli(FILE *fp, int sockfd, const SA *pservaddr, socklen_t servlen){

int n, maxfdp1;const int on = 1;char sendline[MAXLINE], recvline[MAXLINE + 1];fd_set rset;socklen_t len;struct sockaddr *preply_addr;

preply_addr = Malloc(servlen);Setsockopt(sockfd, SOL_SOCKET, SO_BROADCAST, &on, sizeof(on));Pipe(pipefd);maxfdp1 = max(sockfd, pipefd[0]) + 1;FD_ZERO(&rset);


while (Fgets(sendline, MAXLINE, fp) != NULL) {Sendto(sockfd, sendline, strlen(sendline), 0, pservaddr, servlen);

alarm(5);for ( ; ; ) {FD_SET(sockfd, &rset);FD_SET(pipefd[0], &rset);if ( (n = select(maxfdp1, &rset, NULL, NULL, NULL)) < 0) {if (errno == EINTR)continue;elseerr_sys("pselect error");}

if (FD_ISSET(sockfd, &rset)) { len = servlen; n = Recvfrom(sockfd, recvline, MAXLINE, 0, preply_addr, &len); recvline[n] = 0; /* null terminate */ printf("from %s: %s", Sock_ntop_host(preply_addr, servlen), recvline);}

if (FD_ISSET(pipefd[0], &rset)) {Read(pipefd[0], &n, 1); /* timer expired */break;}} } }

Figure 18.10

static voidrecvfrom_alarm(int signo){

Write(pipefd[1], "", 1); /* write 1 null byte to pipe */return;

}

Figure 18.10

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Chapter18 broadcasting